1. Field of the Invention
The present invention relates to an extreme ultra violet (EUV) light source apparatus to be used as a light source of exposure equipment.
2. Description of a Related Art
In recent years, as semiconductor processes become finer, photolithography has been making rapid progress to finer fabrication. In the next generation, microfabrication of 100 nm to 70 nm, further, microfabrication of 50 nm or less will be required. Accordingly, in order to fulfill the requirement for microfabrication of 50 nm or less, for example, exposure equipment is expected to be developed by combining an EUV light source generating EUV light with a wavelength of about 13 nm and reduced projection reflective optics.
As the EUV light source, there are three kinds of light sources, which include an LPP (laser produced plasma) light source using plasma generated by applying a laser beam to a target (hereinafter, also referred to as “LPP EUV light source apparatus”), a DPP (discharge produced plasma) light source using plasma generated by discharge, and an SR (synchrotron radiation) light source using orbital radiation. Among them, the LPP type EUV light source apparatus has the advantages that extremely high intensity close to black body radiation can be obtained because plasma density can be considerably made larger, that light emission of only the necessary waveband can be performed by selecting the target material, and that an extremely large collection solid angle of 2π steradian can be ensured because it is a point light source having substantially isotropic angle distribution and there is no structure surrounding the light source such as electrodes. Therefore, the LPP type EUV light source apparatus is considered to be predominant as a light source for EUV lithography requiring power of more than several tens of watts.
Here, a principle of generating EUV light in the LPP type EUV light source apparatus will be briefly explained. When laser light is applied to a target material supplied into a vacuum chamber, the target material is excited and plasmarized. Various wavelength components including EUV light are radiated from the plasma. Then, the EUV light is reflected and collected by using an EUV collector mirror that selectively reflects a desired wavelength component (e.g., a component having a wavelength of 13.5 nm), and outputted to an exposure unit.
In the LPP EUV light source apparatus, when plasma is generated by applying a laser beam to a target, especially, if debris containing fast ions, neutral particles, and residual droplets emitted from the plasma are left, the operation of the EUV light source apparatus is disturbed. That is, the fast ions scrape (or sputter) the structures within the chamber such as a collector mirror, a nozzle of a target supply unit, and so on, and change the shapes of those structures. When the shape of the collector mirror changes, the reflectance of the collector mirror becomes lower and the output as the EUV light source becomes lower. Further, regarding the other structures, their functions are deteriorated and the operation as the EUV light source is hindered due to shape changes. Furthermore, the neutral particles and the residual droplets of the target are deposited on the structures including the collector mirror within the chamber and decrease the function of the EUV light source apparatus. For example, in a turbo-molecular pump for maintaining the degree of vacuum within the chamber, if the neutral particles and the residual droplets are deposited on turbine blades, the exhaust velocity becomes lower and, in the worst case, the blades collide with each other and are damaged, the turbo-molecular pump breaks down, and the EUV light source apparatus no longer operates.
A technology of protecting structures within the chamber from these fast ions, neutral particles, and/or residual droplets is called a mitigation technology, and various technologies have been proposed and implemented in the process of development of EUV light source. Japanese Patent Application Publication JP-P2006-80255A discloses a technology of protecting a collector mirror by applying a magnetic field into a chamber and ionizing neutral particles, and trapping the charged particles by the applied magnetic field. Further, U.S. Patent Application Publication US 2006/0091109 A1 discloses an EUV light source apparatus in which a reactive gas is introduced into a chamber and the attached material to a collector mirror and so on are allowed to react with the reactive gas and are exhausted.
The mitigation technology conventionally used is intended to prevent the deterioration of the performance of main optical components such as a collector mirror due to attachment of debris flying from the plasma and sputtering by the debris. Accordingly, even if the debris can be prevented from reaching the main optical components such as a collector mirror, the LPP EUV light source apparatus cannot stably be run in a long period. That is, in the EUV light source apparatus, the debris, which are produced from the plasma when a laser beam is applied to a target to generate the plasma for EUV light generation, stay and gradually accumulate within the chamber, and the accumulated debris contaminate the interior of the chamber.
Accordingly, there are problems that the accumulated debris are deposited on the main optical components, and thereby, reduce the reflectance or transmittance of the main optical components and deteriorate the performance of the main optical components, and the laser beam and EUV light are scattered due to increase of debris gradually staying within the chamber and efficient EUV light generation and collection are inhibited. On this account, it has been necessary to clean the interior of the chamber while the generation of EUV light is stopped at regular time intervals. Further, there is a problem that the staying debris enter the pump attached for evacuating the chamber and deteriorate the evacuation performance of the pump, and the operation time of the apparatus becomes shorter for maintenance of the pump. In any case, the long-term operation stability of the LPP EUV light source apparatus is deteriorated by the debris dispersing and staying within the chamber for a long period, and resolving these problems has been an issue.